A multiple-input multiple-output (MIMO) technology is a key technology for the 3rd generation (3G) and 4th generation (4G) mobile communication systems, it can expand system capacity, improve transmission performance and integrate with other physical layer technologies well. However, when channel correlation is strong, diversity gain and multiplexing gain brought by multiplex channels will greatly decline, thus causing a significant drop in MIMO system performance. At present, a new MIMO precoding method has been proposed. This method is an efficient way of MIMO multiplexing, in which an MIMO channel is divided into several independent virtual channels through precoding at the receiving and sending ends, thus effectively eliminating the impact of the channel correlation. Therefore, the precoding technology can guarantee the stability of an MIMO system in various environments.
A long term evolution (LTE) system is an important 3rd generation partnership project. The precoding technology is realized in the LTE by setting a codebook (a precoding matrix set) between a user equipment (UE) and an evolved node base (eNodeB), the UE selects an optimal precoding matrix from the codebook according to a certain criterion (e.g. throughput maximization or the right singular matrix closest to a channel matrix), and feeds back a precoding matrix indicator (PMI) to the eNodeB. The eNodeB finds out the corresponding precoding matrix from the codebook according to the received PMI, and performs precoding transmission during downlink sending by using the precoding matrix; in addition, it is also necessary to feed back rank indication (RI) information over an uplink channel, and the RI information denotes the maximum number of symbols transmitted on one subcarrier.
FIG. 1 is a structure diagram illustrating a basic frame structure in an LTE system. As shown in FIG. 1, the frame structure can be divided into four grades, which are radio frame, half-frame, subframe, and time slot and symbol; wherein one radio frame is 10 ms long and consists of two half-frames; every half-frame is 5 ms long and consists of 5 subframes; every subframe is 1 ms long and consists of 2 time slots; and every time slot is 0.5 ms long.
When normal cyclic prefixes are used in the LTE system, one time slot contains 7 uplink/downlink symbols, each of which is 66.7 us long, wherein the cyclic prefix (CP) of the first symbol is 5.21 us long, and the cyclic prefix of each of other 6 symbols is 4.69 us long.
When extended cyclic prefixes are used in the LTE system, one time slot contains 6 uplink/downlink symbols, each of which is 66.7 us long, wherein the cyclic prefix of each symbol is 16.67 us long.
A resource element (RE) is a subcarrier in an orthogonal frequency division multiplexing (OFDM) symbol, while a downlink resource block (RB) consists of 12 continuous subcarriers and 7 continuous OFDM symbols (if the CP is long, then the number of OFDM symbols is 6), which occupies 180 kHz in the frequency domain and has a time length of a normal time slot in the time domain. FIG. 2 is a structure diagram illustrating resource blocks of bandwidth 5 MHz in an LTE system. As shown in FIG. 2, resource allocation is performed by taking a resource block as a basic unit.
When a target user feeds back RI information, if the target user does not need to send any data, then the RI information is transmitted over a physical uplink control channel (PUCCH); and if the target user needs to send data, then the RI information is transmitted over a physical uplink shared channel (PUSCH).
The PUCCH has six formats, which are format 1, format 1a, format 1b, and format 2, format 2a and format 2b. The format 1 is used for transmitting 1-bit scheduling request (SR) information, denoting whether there is an SR or not; the format 1a is used for transmitting 1-bit single-code-stream acknowledgement/negative acknowledgement (ACK/NACK) information; the format 1b is used for transmitting 2-bit double-code-stream ACK/NACK information, wherein each code stream corresponds to 1-bit ACK/NACK information; the format 2 is used for transmitting channel quality indicator (CQI)/PMI and RI information; the format 2a is used for transmitting CQI/PMI and RI information and single-code-stream ACK/NACK information, and it is used in the scenario that the cyclic prefix is a normal cyclic prefix; the format 2b is used for transmitting CQI/PMI and RI information and double-code-stream ACK/NACK information, and it is used in the scenario that the cyclic prefix is a normal cyclic prefix. FIG. 3 is a diagram illustrating the frequency domain location of a physical uplink control channel in an LTE system. As shown in FIG. 3, every PUCCH occupies resources of two resource blocks; the RI information is 1 bit or 2 bit long, and only the RI information will be sent when the RI information and the CQI/PMI information are sent within a same subframe.
For a normal cyclic prefix and an extended cyclic prefix, the channel structure of the PUCCH format 1 is described as follows.
FIG. 4 is a channel structure diagram of PUCCH format 1 when a normal cyclic prefix is used. As shown in FIG. 4, in a normal cyclic prefix, a constant amplitude zero auto correlation (CAZAC) sequence of length 12 is selected as a basic sequence. One CAZAC sequence is mapped onto 12 frequency domain locations of each symbol in one resource block. Time domain spectrum spread is performed on the CAZAC sequence through a wash orthogonal code of length 4, and the four spread CAZAC sequences are mapped onto symbols (#0, #1, #5, #6) of a time slot; then time domain spectrum spread is performed on the CAZAC sequence through a Discrete Fourier Transform (DFT) orthogonal code of length 3, and the three spread sequences are mapped onto symbols (#2, #3, #4) of the time slot. Wherein the CAZAC sequences after 3 times time domain spectrum spread are used as reference signals (RSs) of the PUCCH, and the CAZAC sequences after 4 times time domain spectrum spread are used for data transmission on the PUCCH.
FIG. 5 is a channel structure diagram of PUCCH format 1 when an extended cyclic prefix is used. As shown in FIG. 5, in an extended cyclic prefix, a CAZAC sequence of length 12 is selected as a basic sequence. One CAZAC sequence is mapped onto 12 frequency domain locations of each symbol in one resource block. Time domain spectrum spread is performed on the CAZAC sequence by using a wash orthogonal code of length 4, and the four spread CAZAC sequences are mapped onto symbols (#0, #1, #4, #5) of a time slot, then time domain spectrum spread is performed on the CAZAC sequence by using a wash orthogonal code of length 2, and the two spread sequences are mapped onto symbols (#2, #3) of the time slot. Wherein the CAZAC sequences after 2 times time domain spectrum spread are used as RSs of the PUCCH, and the CAZAC sequences after 4 times time domain spectrum spread are used for data transmission on the PUCCH.
Every PUCCH format 1 channel corresponds to a frequency domain location (resource block index), the above-mentioned basic sequence and the time domain spread spectrum code, with which every PUCCH format 1 channel can be determined. Whether to send the PUCCH format 1 channel or not determines whether there is SR information.
In the LTE system, uplink data are sent by way of single-carrier frequency-division multiple access (SC-FDMA), uplink resources to be mapped are therefore required to be continuous, and all data sent in every symbol, after being modulated, will go through DFT and be mapped to corresponding frequency domain locations. Moreover, if two or more PUCCH formats are sent within a same subframe, inter-symbol interference will happen and the orthogonality of codes between PUCCHs can not be guaranteed, so the system performance will decline. At present, in the existing technologies no specific solution has been proposed for a method for sending RI information and SR information within a same subframe.